Replace sphinx-tabs with sphinx-design

[Copilot]

sphinx-tabs is incompatible with recent Sphinx versions, causing documentation build failures. sphinx-design provides equivalent functionality and was already installed.

Changes

• pages_requirements.txt: Removed sphinx-tabs dependency
• docs/conf.py: Removed sphinx_tabs.tabs extension
• docs/guide/installation.rst: Updated tab directive syntax

Syntax Migration

-.. tabs::
+.. tab-set::

-   .. tab:: Conda
+   .. tab-item:: Conda

       .. code-block:: console

          $ conda install -c conda-forge gwpopulation

All tab functionality remains identical; only the directive names changed.

Original prompt

The documentation building job has started failing because sphinx tabs is not compatible with recent versions of sphinx. Tabs can now be specified using sphinx-design. Switch all uses of sphinx-tabs to use sphinx-design instead.

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[ColmTalbot]

Looks good to me assuming the docs job passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.15%. Comparing base (5054826) to head (3157e2e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #138   +/-   ##
=======================================
  Coverage   95.15%   95.15%           
=======================================
  Files          14       14           
  Lines         907      907           
=======================================
  Hits          863      863           
  Misses         44       44           

Flag	Coverage Δ
python3.10	95.15% <ø> (ø)
python3.11	95.15% <ø> (ø)
python3.12	95.15% <ø> (ø)
python3.13	95.15% <ø> (ø)

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